Windows provide a significant design challenge for architects and builders. On the one hand, large, high transmission windows are desirable for providing an "open air" feeling to building occupants, significant solar or passive heat gain when the outdoor or ambient temperature is low and windows are desirable for allowing visible light into a building or structure to provide daylighting and thereby substantially reduce the need for electric lighting during the time when occupants are most often present. On the other hand, windows can allow in excessive amounts of solar heat when the outdoor or ambient temperature is high and air cooling or conditioning is already in use. This is especially true at certain times of the day and certain parts of a building when the solar radiation is shining directly on or through the windows.
An attempted solution to the problem is the use of window treatments like shades, drapes and blinds. However these are expensive, cumbersome and aesthetically undesirable solutions, particularly in large office buildings, hotel atria and public structures like airport terminals. Energy efficient windows based on thermochromic, photochromic, electrochromic and photoelectrochromic technologies have been proposed for providing variable transmission windows for use in buildings and structures. However, until now, none have shown the performance, durability, cost effectiveness and convenience to be used on a commercial basis.